Determining the position and direction of a conductive pipe (metallic casing, for example) accurately and efficiently is required in a variety of downhole applications. Perhaps the most important of these applications is the case of a blown out well in which the target well must be intersected very precisely by a relief well in order to stop the blowout. Other important applications include drilling of a well parallel to an existing well in Steam Assisted Gravity Drainage (“SAGD”) systems, avoiding collisions with other wells in a crowded oil field where wells are drilled in close proximity to each other and tracking an underground drilling path using a current injected metallic pipe over the ground as a reference.
Conventional approaches have attempted to provide solutions to this problem. In one method, an electrode type source is used to induce current on the target casing to thereby generate a magnetic field. Gradient of the magnetic field radiated by the target casing, in addition to the magnetic field itself, is measured using two receivers which often require in-situ calibration. By using a relationship between the magnetic field and its gradient, an accurate ranging measurement is made. However, since two receivers are utilized, in-situ calibration of the receivers is often required.